The invention relates in particular to AZS (alumina-zirconia-silica) products which are less expensive than the conventional AZS products currently available and whose properties are adapted to non-extreme service conditions such as those prevailing in the rear areas of glass-melting furnaces and/or in some furnace superstructures. The products of the invention contain more silica and less zirconia than the conventional AZS products currently used.
Products which are fused and cast in a mould (also called fused cast products) can be obtained by melting a mixture of appropriate raw materials in an electric arc furnace or any other melting technique suitable for these products. The molten liquid is then cast in a mould to obtain shaped parts directly. In general, the product is then subjected to a controlled cooling cycle in order to bring it to ambient temperature without breakage. This operation is called annealing by those skilled in the art.
AZS products have been known for several decades. U.S. Pat. No. 2,438,552 describes one of the first improvements made to this type of product. It advocates the addition of Na2O (1-2.2%) and MgO/CaO (0.2-0.8%) to solve feasibility problems concerning products containing 45-70% Al2O3, 14-40% ZrO2 and 9-12% SiO2.
The AZS products currently on sale, such as the Applicant""s ER-1681, ER-1685 or ER-1711, contain from 45 to 50% Al2O3, from 32 to 40% ZrO2, from 12 to 16% SiO2 and approximately 1% Na2O.
These products are entirely suitable for the production of glass-melting furnaces. The present AZS materials are principally used for the zones in contact with the molten glass as well as for the superstructure of the glass-melting furnaces. However, the superstructures of some laboratory furnaces are less demanding from the point of view of corrosion resistance.
In addition, the rear areas of the furnace, such as the burner flues or the tops, walls and stacking parts of heat regenerator chambers are areas which, not being in contact with the molten glass, are less demanding from the point of view of corrosion resistance. The present AZS products are little used in these areas because of their excessively high cost.
On the other hand, materials in these areas of the furnace are subject to intense variations in temperature caused by the operating cycles of the regenerators. In fact, during operation of the stacking chambers of the regenerator, hot gases from the furnace enter the stack at the top and release their heat energy. During this time, cold air enters at the bottom of another stack heated during the preceding cycle and is hot when it exits the top of the stack, from where it is conducted to the burners.
There thus exists a need for a cheap refractory material whose properties are suited to operating conditions less demanding than those which prevail in the rear areas of glass-melting furnaces and/or in some superstructures of less efficient furnaces.
With the aim of proposing a less expensive product, we have envisaged reducing the cost of the raw materials. This may be achieved firstly by reducing the proportion of zirconia, which is an expensive component of the composition, and secondly by using secondary materials (manufacturing waste or recycled spent materials).
However, this attractive solution entails a number of problems concerning feasibility and behaviour of the materials in service, associated with the change in the chemical analysis of the materials and the possible presence of high levels of impurities which may, for example, arise from the use of spent products.
We have found, surprisingly, a field of chemical analysis for ensuring the feasibility of the parts as well as good behaviour of the material in service in the abovementioned areas of the glass-melting furnaces.
The invention more particularly provides fused and cast AZS products characterized in that they contain:
Al2O3: 45-65 wt. %
ZrO2: 10.0-29.0 wt. %
SiO2: 20.0-24.0 wt. %
SiO2/(Na2O+K2O) (weight ratio):4.5-8.0
other species: 0.5-4.0 wt. %
The SiO2/(Na2O+K2O) weight ratio is preferably in the range from 6.0 to 7.0.
The ZrO2 proportion is preferably in the range from 14.0 wt. % to 25.0 wt. %.
The Al2O3 proportion is preferably in the range from 50 wt. % to 65 wt. %.
AZS products according to the invention that are particularly preferred contain:
Al2O3: 50-65 wt. %
ZrO2: 14.0-25.0 wt. %
SiO2:  greater than  20.0-24.0 wt. %
SiO2/(Na2O+K2O) (weight ratio):6.0-7.0
other species: 0.5-4 wt. %
The AZS products according to the invention can be moulded into any desired shape. Preferred shapes are stackable cruciform components and blocks. The blocks preferably have a ZrO2 proportion which is in the range from 20 wt. % to 25 wt. %. Such blocks are particularly useful in the superstructures or the rear areas of glass-melting furnaces.
The cruciform components preferably have a ZrO2 proportion which is in the range from 14 wt. % to 20 wt. %. Such components are particularly useful in glass-melting furnace regenerators.